Whitlockite

Whitlockite
An example of the mineral Whitlockite on display at the Royal Ontario Museum.
General
Category	Phosphate minerals
Formula (repeating unit)	Ca9(Mg,Fe++)(PO4)6(PO3OH)
Strunz classification	8.AC.45
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) H-M symbol: (3m)
Space group	R3c
Unit cell	a = 10.33, c = 37.103(5) [Å]; Z = 3
Identification
Color	Colorless, gray-white, light pink, light yellow
Crystal habit	rhombohedral crystals, often tabular, massive, microcrystalline crusts and as "cave pearls"
Cleavage	None
Fracture	Brittle
Tenacity	Brittle
Mohs scale hardness	5
Luster	Vitreous to resinous
Streak	White
Diaphaneity	Transparent
Density	3.13
Optical properties	uniaxial (-)
Refractive index	nω = 1.629 nε = 1.626
Birefringence	δ = 0.003
Other characteristics	Piezoelectric and pyroelectric
References

Whitlockite is a mineral, an unusual form of calcium phosphate. Its formula is Ca₉(MgFe)(PO₄)₆PO₃OH. It is a relatively rare mineral but is found in granitic pegmatites, phosphate rock deposits, guano caves and in chondrite meteorites. It was first described in 1941 and named for Herbert Percy Whitlock (1868–1948), American mineralogist, Curator, American Museum of Natural History, New York City, NY, US.

With regards to periodontal dentistry, magnesium whitlockite comprises one component of many of the inorganic content of calculus. It is found primarily in subgingival calculus (as opposed to supragingival calculus). It is also found more in posterior as opposed to anterior regions of the oral cavity.

Whitlockite is a member of the phosphate group of minerals with three distinct occurrences. For many years, these occurrences were thought to be identical. However, recent studies using x-ray and electron diffraction have been able to identify compositional differences that separate one type of whitlockite from another. There are two inorganic occurrences of whitlockite that differ chiefly by the presence or absence of hydrogen. This difference was not initially observed due to technical limitations, such as small crystal size. Although the identity of the “true” whitlockite is still debated, efforts are now being made to officially distinguish terrestrial whitlockite from its phase in meteorites as two distinct minerals. Whitlockite can also be found in different types of biological deposits. Organic instances of whitlockite are virtually identical in composition, but typically contain magnesium, which further distinguishes them from inorganic instances of this mineral. Magnesium whitlockite has been implicated in different disease states and is currently being studied for use in the fabrication of human prosthetics.